add minimal docstring to mex files

The following compiled files generate Octave documentation warings

warning: doc_cache_create: unusable help text found in file 'gpml_demoRegression'
warning: doc_cache_create: unusable help text found in file 'isLegal'
warning: doc_cache_create: unusable help text found in file 'precondDiag'
warning: doc_cache_create: unusable help text found in file 'precondTriu'
warning: doc_cache_create: unusable help text found in file 'precondTriuDiag'
warning: doc_cache_create: unusable help text found in file 'lbfgsAddC'
warning: doc_cache_create: unusable help text found in file 'lbfgsC'
warning: doc_cache_create: unusable help text found in file 'lbfgsProdC'
warning: doc_cache_create: unusable help text found in file 'lbfgsb'
warning: doc_cache_create: unusable help text found in file 'mcholC'

get rid of these warnings by adding a minimal docstring, e.g. "internal function, not documented"